1. Field of the Invention
This invention relates to a process and apparatus for combustion of a combustible material such as coal, wood, wood waste, municipal solid waste, refuse derived fuel, sludge, biomass or other combustible material in which a mixture of recirculated flue gases and flyash with organic content is injected into a combustion chamber to provide a region of reduced oxygen availability above a primary combustion zone, and in so doing, provide an atmosphere in the combustion chamber which not only limits NO.sub.x formation in the secondary combustion zone, but also reduces NO.sub.x after it has formed.
2. Description of Prior Art
Most of the existing processes and apparatuses for combustion of combustible material such as municipal solid waste, refuse derived fuel, wood, coal, or sludge include a combustion chamber equipped with a sloped or horizontal stoker grate that reciprocates, travels, or vibrates to move the combustible material from the combustible material inlet side of the combustor to the ash removal side of the combustor. A portion of the combustion air, generally equivalent to 1.0 to 1.3 of the combustible material stoichiometric requirement, is supplied under the stoker grate. Such combustion air is typically called undergrate air and is distributed through the stoker grate to dry and burn the combustible material present on the stoker grate. The combustible material is first dried on the drying portion or drying grate of the stoker grate, then combusted on the combustion portion or combustion grate of the stoker grate. The residual material that primarily includes ash and carbon is then decarbonized or burned on the burnout portion or burnout grate of the stoker grate. The bottom ash is then removed through an ash pit. To assure carbon burnout, a high level of excess air, compared to the amount required for carbon burnout, is maintained at the burnout grate. In addition to other species, the products of combustible material drying, combustion and burnout contain products of incomplete combustion such as carbon monoxide and total hydrocarbons, oxides of nitrogen, such as NO, NO.sub.2, N.sub.2 O and other nitrogen-bearing compounds such as NH.sub.3, HCN and the like.
There is a wide range in size for the various combustible materials in the form of solid fuels burned on stokers as well as a wide range of burning characteristics for these fuels. In the case of coal-fired stoker boilers, the coal feed is approximately sized minus 21/2" to plus 1/8". Some of the coal feed is smaller than 1/8" and typically is burned in suspension and the flyash carried out of the furnace by the combustion gases.
Typical stoker boilers have one or more stages of mechanical separation for flyash. This flyash can be collected and disposed of, or injected back into the boiler to improve efficiency of fuel utilization. The collection and disposal of flyash is taught, for example, by U.S. Pat. No. 5,052,312 which discloses an apparatus and method utilizing a cyclone furnace to vitrify inorganic elements generated from the incineration of waste materials including contaminated soils into an inert vitreous slag without releasing further pollutants into the atmosphere. Also taught is the combination of a cyclone furnace with a boiler so that the cyclone furnace receives the ash from the boiler and converts the ash to a vitreous slag suitable for disposal in a conventional landfill. U.S. Pat. No. 4,185,080 teaches a method for reducing the sulfur oxide content of combustion gases resulting from combustion of sulfur-containing fossil fuels in which the sulfur-containing fossil fuels are combusted in the presence of calcium carbonate or calcium magnesium carbonate and the sulfur oxides formed by the combustion react with the calcium oxide or calcium magnesium formed from the carbonate during the combustion to form calcium sulfate or calcium magnesium sulfate. Ash formed during the combustion process is removed from the oxides by sifting. And U.S. Pat. No. 5,429,059 teaches a coal-fired firetube boiler and method for converting a gas-fired firetube boiler to a coal-fired firetube boiler in which ash is collected in a bag house from which it is dropped to an ash receptacle for subsequent disposal, and soot blowers are connected to the individual boiler tubes for removal of particulates from the boiler tubes. Finally, U.S. Pat. No. 5,311,829 teaches a process for reducing sulfur emissions and increasing particulate removal in the combustion of pulverized coal in which pulverized sulfur-containing coal is injected with combustion air into a combustion chamber and fired to create a combustion zone within the chamber, thereby forming gaseous sulfur and particulate emission products. The emission products are exhausted and the particulate emission product removed therefrom.
Flyash typically comprises a significant amount of organic content and energy lost in flyash from, for example, coal-fired stoker boilers may represent as much as 8% of the heat input to the boiler. The flyash can contain 80-85% carbon and may contain a significant amount of fuel-bound nitrogen, in which case the flyash more closely resembles the feedstock from a pulverized coal combustion boiler. Thus, to improve fuel utilization, many stoker boilers have provisions for flyash reinjection. Typically, where reinjection of flyash is utilized, the flyash from the dropout section of the boiler bank and the cyclone separators is injected back into the boiler using air. The flyash is typically sized 93% minus 20 mesh to plus 100 mesh. The problem with injection of these fines using air is that the fines bum quickly and hot, thereby generating thermal and fuel NO.sub.x.